It is distinguished from malum prohibitum, which refers to acts that are wrong only because they are prohibited by law.
In contrast, malum prohibitum crimes are criminal not because they are inherently bad, but because the act is prohibited by the law of the state.
For example, most United States jurisdictions require drivers to drive on the right side of the road.
The question between inherently wrong versus prohibited most likely originated in Plato's Socratic dialogue, Euthyphro, in which Socrates famously asked "Is the pious (τὸ ὅσιον) loved by the gods because it is pious, or is it pious because it is loved by the gods?"
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